Chair back



FIG. 1 is a perspective view of a chair back in accordance with our new design, with portions of a chair forming the environment of the chair back and being illustrated in broken lines, which broken lines are for illustrative purposes only and form no part of the claimed design;

FIG. 2 is a front elevational view of the chair back shown in FIG. 1;

FIG. 3 is a right-side elevational view thereof, the right and left sides of the chair back being a mirror image;

FIG. 4 is a front view of FIG. 1;

FIG. 5 is a top view;

FIG. 6 is a rear view; and,

FIG. 7 is a bottom view. 

The ornamental design for a chair back, as shown and described. 